This invention relates generally to closed cycle engines having pistons reciprocating within cylinders, and particularly relates to an improved sealing piston ring assembly located between a piston and a cylinder wall. The invention is particularly useful in a Stirling engine.
A Stirling engine is a closed cycle engine. This means that during every cycle of the engine, the same working medium namely, a gas such as air, hydrogen or helium, is compressed, cooled, heated and expanded. In such an engine, good piston seals are a necessity.
In a Stirling engine, internal seals cannot be lubricated with oil because oil will affect the cycle. A dry piston ring seal is thus required. In such seals, it is known to employ a material known as "Rulon" which is a trademark identifying a heat stable plastic polymer of fluorine and ethylene mixed with glass powder and other materials. Rulon is a solid material that has a very low coefficient of friction and performs well as a seal engaging metal.
However, in double acting engines, such as a Stirling engine, simple piston rings formed of Rulon have proved to be unsatisfactory because a leak in one direction always results in the development of pressure differentials of unequal magnitudes. When two Rulon piston rings are employed, higher friction is encountered.
The principal object of this invention, therefore, is to provide an improved sealing piston ring assembly for Stirling engines in which an annular metal piston ring member cooperates with pairs of thin annular sealing ring members formed of low friction material, such as Rulon, and positioned on opposite end faces of the metal piston ring member so that when the piston is moving in one direction, one of the sealing ring units seals against the piston ring groove and when the piston is moving in the opposite direction, the other sealing ring unit seals against the other side of the piston ring groove. The metal piston ring member is maintained in a clearance relation with the cylinder wall and the sealing ring units are formed of very thin annular members so that they do not create significantly high frictional forces on the cylinder wall.
The result is a very effective piston ring assembly that forms a seal on the opposite sides of the piston and which can be operated with a minimum of frictional interference.